The pulp and paper industry produces paper for documents, books, newspapers and the like, and heavier grades for packaging, corrugated paper, shipping containers and the like. The most important source of fiber for these paper products is cellulose, which is derived from wood. Wood is generally classified as either softwood, which provides long-fibered pulps, and hardwood, which provides short-fibered pulp. Pulping operations generally fall within one of three broad categories, namely, operations producing mechanical pulp (groundwood and thermomechanical pulps), chemical pulp (sulphate, sulphite, soda, Kraft and semi-chemical pulps) and secondary fiber pulp (reclaimed paper pulp). Most, but not all, of the wood intake of the soda, Kraft and semi-chemical pulping processes is hardwood. Most, but not all, of the wood intake for the mechanical, sulphate and sulphite pulping processes, is softwood. Pulp is bleached or not generally depending on its intended end-use. In general, mechanical pulps are seldom bleached, while chemical pulps are often, but not always, bleached. The pulp used for most types of products are blends of pulps from different pulping processes, and such blends may include both bleached and unbleached pulps. For instance, carton board may be formed from mainly unbleached mechanical pulp, with minor proportions of bleached soda pulp and bleached semi-chemical pulps, while soft tissue paper may be formed from mainly bleached sulphite and sulphate pulps, with minor portions of bleached soda pulp and unbleached semi-chemical pulp.
About half of the weight of timber is cellulose. Without the bark, depending on the species, the trunk of a tree consists of from about 65 to about 85 percent fiber, bound together with from about 15 to about 35 percent lignin. The pulping process separates the fibers preparatory to their reintegration in the final product. Logs are first debarked, by which about 7 to 9 percent of their weight is removed. The cleaned logs are then pulped either mechanically or, after being cut into chips, chemically.
In the production of the mechanical pulp known as groundwood pulp, debarked logs are generally loaded into the magazine of a grinder in which they are pressed against a grinding wheel to separate the bundles of fibers into individuals strands. Except for a small percentage of organic matter extracted during the grinding process, the lignin, a noncarbohydrate portion of wood that held the fibers together, remains in the finished pulp. The paper produced by mechanical pulping is used for the manufacture of impermanent papers, such as newsprint, catalogs, magazines, paperboard and the like, often as a blend with some chemical pulp to increase the paper strength to the degree required for the given printing press and end use.
Another process for producing mechanical pulp is thermomechanical pulping wherein wood chips are washed with recycled water, then macerated in a screw press to a homogeneous slush, and passed continuously through a steam-heated digester. The recycled water may be white water or filtrate that contains pulp chemical residues. This treatment softens the fibers and permits them to separate somewhat from lignin, which improves refining and produces a mechanical pulp superior to groundwood pulp in strength. Refining is conducted in three stages following the digester. The refined pulp is screened, cleaned, and then may be adjusted to the required consistency for bleaching. (Hydrosulfite and hydrogen peroxide are the bleaching chemicals used for both groundwood and thermomechanical pulps.)
Chemical pulping employs chemicals to soften or dissolve the lignin and other organic materials holding the fibers together, so as to release the fibers without extensive mechanical working. This softening or dissolving process is known as digestion. The basic chemical processes are classified as acid, neutral or alkaline pulp processes. The acid pulping processes include those designated acid sulfite, sodium base, ammonium base, calcium base and magnesium base. The neutral processes include those designated neutral sulfite, neutral sulfite-semichemical, and chemiground. The alkaline processes include those designated Kraft and Kraft-semichemical. The Kraft process, which uses a mixture of sodium hydroxide and sodium sulfite, is also known as "sulfate" process. The soda process, used in very few mills, uses sodium hydroxide alone as the alkaline agent. In the chemiground and Kraft-semichemical processes, liquor is used to soften the wood before grinding. In all of the chemical pulping processes, a chemical solution is fed to a digester, in which it is mixed with wood that has been cut into chips to permit the liquor to penetrate effectively and produce a uniform pulp. The mixture is then cooked for a specified period at the optimum temperature for the particular process and the type of wood being pulped. Most digesters operate on a batch basis, but continuous digestion processes are increasingly coming into use.
Untreated wood generally contains some amount of pitch, which is typically located in parenchyma cells and on the surfaces of the fiber. Based on solubility in ethyl ether values, pitch may comprise from about 0.7 to about 2.4 weight percent of hardwoods such as beech and white birch, and from about 0.7 to about 4.3 weight percent of softwoods such as eastern hemlock and jack pine, based on the total weight of unextracted (oven-dry) wood.
The term "pitch" refers to a variety of naturally occurring, hydrophobic, organic resins of low and medium molecular weight, and to the deposits these resins cause during the pulping and papermaking processes. Pitch includes fatty acids, resin acids, their insoluble salts, and esters of fatty acids with glycerol (such as the triglycerides) and sterols, as well as other fats and waxes. These compounds display characteristic degrees of temperature-dependent viscosity, tackiness, and cohesive strength. They may deposit alone or together with insoluble inorganic salts, filler, fiber, defoamer components, coating binders, and the like.
Pitch deposits may occur throughout a pulp or paper mill and these deposits can both degrade product quality and impair production rates. They can impair production rates by decreasing the efficiency of pulp washing, screening, centrifugal cleaning, and refining, and disrupting many paper machine operations. Pitch can degrade the product paper by causing spots, holes, picking, and scabs in the final paper product or sheet.
Present paper production trends are likely to increase pitch deposit problems, unless such problems are counteracted by more effective control methods. Such present production trends include the use of: high-speed machines that create high shear rates and greater pitch deposits; higher production rates that increase the load on washing equipment and thus increase the concentration of pitch in the stock; defoamer chemicals which may aggravate pitch deposition problems; high-yield chemical and mechanical pulps that often contain more resinous pitch materials; and the reuse of white water, and more complete closure, which concentrate pitch and aggravate pitch deposition, particularly in bleach plants. These trends all increase the potential for pitch deposition problems, and the severity of such problems when they occur. High-quality paper products, however, must be virtually free from pitch-related defects.
Past efforts to control pitch problems are widely varied. Common pitch control measures include aging or seasoning wood, the use of wood species with low resin contents, and the modification of pulping parameters. Modifications in pulping parameters include process variables such as pH, temperature, first-pass retention, washing efficiency, bleaching agent and the like. Modifications in pulping parameters also includes the use of process additives, such as dispersants, cationic polymers, alum, and talc, all of which have been employed to control pitch problems.
The composition of the pitch is a major factor in the amount of pitch that deposits and the characteristics of the deposits. Pitch composition varies depending on the season and the type of wood and thus some pitch problems appear only in certain months of the winter and spring, and some wood species create greater pitch problems during pulping and papermaking than other species. The nonpolar, hydrophobic components of pitch, particularly the triglycerides, in a given pitch composition are considered the major factors as to whether or not the presence of such pitch will lead to pitch deposits. Deposit-forming pitch always contains a significantly higher concentration of triglyercides than pitch that forms no deposits. As further verification of this relationship, it has been determined that triglycerides decompose during seasoning (storage of cut wood before use), and seasoning, as noted above, is a known technique for reducing pitch deposits. Seasoning requires storage of logs for long periods, and thus creates time delays between the harvesting of the logs and their use, and requires a substantial amount of storage space. Moreover, seasoning often leads to decreased pulp brightness. Therefore decomposition of the triglyceride components of pitch by seasoning is often impossible or impractical. A pitch control method that alleviates the need to season wood is considered extremely desirable in the pulp and papermaking field.
It is an object of the present invention to provide a pitch control method that selectively acts upon the triglyceride components of the pitch. It is an object of the present invention to provide a pitch control method that removes not only triglycerides, but also the hydrolysates of triglycerides, from the aqueous phase of a cellulosic slurry. It is an object of the present invention to decompose the triglycerides of pitch without the long storage time, and extensive storage space, required by the seasoning method. It is an object of the present invention to provide such a pitch control method that may be used both for mechanical and chemical pulps. It is an object of the present invention to provide pitch control without introducing undesirable treatment chemicals into the aqueous system of the pulp and papermaking process. These and other objects of the invention are described in more detail below.